1. Field of the Invention
The present invention relates to a three-phase sine wave signal synthesizing circuit and a brushless motor drive circuit using the same
2. Background Prior Art
There have been known various three-phase sine wave signal synthesizing circuits to be used for brushless motor circuits including one disclosed in Japanese Patent (Tokkai) No. 60-96191 and illustrated in FIG. 6 of the accompanying drawings. With the three-phase sine wave signal synthesizing circuit of FIG. 6, whenever detectors 1, 2, which are Hall devices, detect a substantially sinusoidal two-phase output generated by a signal generator 3 comprising a rotor magnet of a brushless motor, they, in turn, generate substantially sinusoidal two-phase output signals Ain and Bin having a phase difference of 120.degree. from each other in terms of electric angle. These output signals Ain and Bin are amplified by amplifiers 4 and 5 and provided respectively as a first-phase output signal Aout and a second-phase output signal Bout. The output signals from the amplifiers 4 and 5 are then added and inverted by a synthesizer 8 comprising an adder 6 and an inverter 7 to produce a third-phase output signal Cout, the first-phase output signal Aout from the amplifier 4, the second-phase output signal Bout from the amplifier 5 and the third-phase output signal Cout from the synthesizer 8 having a phase difference of 120.degree. from one another.
Since a three-phase sine wave signal synthesizing circuit as described above uses a synthesizer 8 for producing a third-phase output signal, the overall circuit configuration inevitably becomes a complicated one, which in turn raises the manufacturing cost, and the accuracy of output signals of the circuit can be relatively poor because of errors involved in substantially sinusoidal three-phase output signals Aout, Bout and Cout.
A high cost of a three-phase sine wave signal synthesizing circuit having a relatively complicated configuration as described above results as a matter of course in a high manufacturing cost of a brushless motor driving circuit using such a synthesizing circuit. Moreover, the poor accuracy of substantially sinusoidal three-phase output signals Aout, Bout and Cout due to the use of a synthesizer 8 results in an uneven torque of the brushless motor.
It is therefore an object of the present invention to provide a three-phase sine wave signal synthesizing circuit which is free from the above disadvantage and has a simplified circuit configuration, requiring a reduced manufacturing cost, and a brushless motor drive circuit using such a three-phase sine wave signal synthesizing circuit.